This invention relates to the field of microwave circuits and more particularly to transmit/receive (T/R) switches for microwave circuits.
As is well known in the art, T/R switches for microwave circuits have a wide variety of applications. In one such application, an amplifier has an output coupled through a transmission line to an output port, the output port being additionally coupled to other circuits. When the amplifier is in use, a microwave signal amplified thereby is coupled to the output port by the transmission line. When the amplifier is not in use, it is desirable to electrically decouple the amplifier and transmission line associated therewith from the output port to prevent signals from the other circuits from coupling into the amplifier circuit. Such electrical decoupling is typically achieved through the use of a T/R switch. One known T/R switch comprises a depletion mode field-effect transistor (FET) having a grounded source electrode. The drain electrode is coupled to the junction of the amplifier's output and the transmission line. A control signal feeds the gate electrode to maintain the FET in a nonconducting condition when the amplifier is in use and to switch the FET to a fully conducting condition when the amplifier is not in use. The FET in the nonconducting condition maintains a high impedance between the drain and source electrodes and thus permits the amplified microwave signal to be coupled to the output port by the transmission line. Alternately, when the FET is switched to the fully conducting condition the impedance between the drain and source electrodes is switched to a low impedance, thus effectively placing ground potential at the output of the amplifier. The transmission line coupled between the output of the amplifier and the output port has an electrical length corresponding to a quarter-wavelength at the nominal frequency of the microwave signal and thus transforms the ground potential at the amplifier end of the transmission line to a high impedance at the output port, thereby electrically decoupling the amplifier and transmission line from the output port when the amplifier is not in use.
Such an arrangement finds application in microwave transmitter/receiver (transceiver) modules as are used in, for example, phased array antennas. A typical transceiver comprises a transmitter channel including a transmitter amplifier coupled between first and second ports by a pair of quarter-wavelength transmission lines, and a receiver channel including a receiver amplifier coupled between the first and second ports by a pair of quarter-wavelength transmission lines. For the transceiver to operate efficiently, it is desirable that the channels be electrically decoupled from one another. Thus, the T/R switching arrangement previously described is utilized in both channels to electrically decouple the receiver channel from the first and second ports during transmitter channel operation and alternately to electrically decouple the transmitter channel from the first and second ports during receiver channel operation. Typically, a transceiver T/R switching arrangement comprises two pairs of grounded source electrode FETs, a first pair associated with the transmitter amplifier and a second pair associated with the receiver amplifier. The transistors in each pair have drain electrodes connected to the input and output terminals, respectively, of the amplifier associated therewith. The gate electrodes of the first pair of FETs are fed by a transmit control signal and the gate electrodes of the second pair of FETs are fed by a receive control signal. During transmission, the transmit control signal biases the gate electrodes of the first pair of FETs to the FET pinch-off voltage and the receive control signal biases the gate electrodes of the second pair of FETs to zero volts. During reception, the converse is true; that is, the transmit control signal biases the gate electrodes of the first pair of FETs to zero volts and the receive control signal biases the gate electrodes of the second pair of FETs to the FET pinch-off voltage. In either phase of operation, the pair of FETs biased at the pinch-off voltage are maintained in an essentially nonconductive condition between their drain and source electrodes, thus presenting a high impedance to ground to the input and output terminals of the amplifier associated therewith and permitting proper operation of that channel. On the other hand, the pair of FETs biased at zero volts are fully conducting between their drain and source electrodes, thus placing essentially a short-circuit to ground at the input and output terminals of the amplifier associated therewith. Thus, the pair of transmission lines coupling that amplifier between the first and second ports are shorted to ground at the amplifier ends thereof. Since each transmission line has an electrical length corresponding to a quarter-wavelength at the nominal operating frequency of the transceiver, the short circuits to ground at the amplifier ends of such pair of transmission lines are transformed by such transmission lines to high impedances at the first and second ports, thereby electrically decoupling that channel from the first and second ports.
While the T/R switching arrangement just described performs satisfactorily in some applications, there are inherent disadvantages associated with it. For example, the drain electrode of the FET coupled to the output terminal of the transmitter amplifier is exposed to relatively high microwave voltages during transmission. As discussed, such FET is pinched-off during transmission and must remain pinched-off for all power levels of the transmitted microwave signal. Typically, the T/R switch FETs are n-channel devices. Thus, during positive instantaneous microwave voltage swings at the transmitter amplifier's output, the gate electrode need merely be more negative than the FET pinch-off voltage to keep the FET pinched-off because the FET's grounded source electrode is also the effective source terminal for the device. However, during negative instantaneous microwave voltage swings at the transmitter amplifier's output, the FET's drain electrode becomes more negative than the grounded source electrode and thus becomes the effective source terminal for the device. Accordingly, for the FET to remain pinched-off, the transmit control signal must maintain the gate electrode voltage more negative than the negative instantaneous microwave voltage swings of the transmitted microwave signal by at least the pinch-off voltage. Hence, a large instantaneous gate-to-drain voltage exists during positive instantaneous microwave voltage swings. The FET must have a gate-to-drain breakdown voltage greater than this instantaneous voltage to survive the operation. Since typical FET gate-to-drain breakdown voltages are limited, the microwave transmission power capability of transceivers using this type of T/R switching is correspondingly limited. Additionally, the pinched-off pair of switching FETs slightly conduct current between their drain and source electrodes, thus loading somewhat the channel associated therewith. This causes insertion loss in that channel, with concomitant loss in microwave signal power.